1. Field of the Invention
The present invention relates to a developing device used for electrophotographic type copiers and printers. More particularly, it relates to a developing device of a vertical circulation type wherein each screw is arranged up and down or upper, middle, and lower so that developer should circulate smoothly and unevenness of toner-density problem should be solved quickly.
2. Description of the Related Art
As an example of a conventional developing device of vertical circulation type, Japanese Unexamined Patent Publication No. 5-333691 here will be referred to. FIGS. 44 and 45 illustrate the structure of a conventional developing device disclosed in the publication. In a developing device 900 according to the publication, a developer reservoir 903 for supplying and a developer reservoir 906 for stirring are arranged up and down. A partition wall 904 is arranged between the developer reservoir 903 and the developer reservoir 906 whereby the partition wall 904 separates the developer reservoirs 903 and 906.
An upper auger 902 is arranged in the developer reservoir 903 and transports developer from left to right in FIG. 45. A lower auger 905 is arranged in the developer reservoir 906 and transports developer from right to left in FIG. 45. That is, transporting direction of the auger 902 and that of the auger 905 are opposite to each other.
The lower auger 905 delivers developer to the upper auger 902 at downstream end 911 for the developer reservoir 906. The upper auger 902 delivers developer to the lower auger 905 at downstream end 912 for the developer reservoir 903. Thereby, developer is circulated clockwise in FIG. 45 and stirred. As apparent from FIG. 44, the upper auger 902 supplies developer to a developer carrier 907. Then, the lower auger 905 collects developer remaining on the developer carrier 907 after development.
However, the developing device 900 has the following problems.
Firstly, in the developing device 900, the relation between developer-transporting capability of the upper auger 902 and that of the lower auger 905 is not considered at all. Therefore, there is a fear that the balance between developer-transporting capability of the upper auger 902 and that of the lower auger 905 may not be found. When developer-transporting capabilities of the augers 902 and 905 do not find a balance, developer does not circulate smoothly. Thereby, developer receives an excessive load and developer is likely to deteriorate soon. Also, thereby, distribution of developer within the developing device becomes imbalance and image-density may be uneven.
Secondly, in the developing device 900, both the developer reservoirs 903 and 906 are separated by the partition wall 904 except the downstream ends 911 and 912. Thereby, portions where the augers 902 and 905 can deliver developer to each other are limited to the downstream ends 911 and 912. Therefore, developer circulates clockwise only, in actual. That is, developer can hardly move from one of the augers to other auger while being transported horizontally by the augers 902 and 905.
Thus, in case that toner-density of developer becomes uneven at some parts, developer keeps circulating for some time with its toner-density uneven. In other words, it takes time for developer to recover from unevenness of toner-density in the clockwise circulation. This also causes unevenness of image-density.
Thirdly, as described, since portions where developer can be delivered up and down are limited to the downstream ends 911 and 912, balance of developer to be delivered between the upper auger and the lower auger is lost in case that the developing device 900 is not installed horizontally. As a result, developer is likely to concentrate in either the developer reservoir 903 or the developer reservoir 906. Thereby, a motor M for driving the augers 902 and 905 has a large load and problem such as step-out occurs to the motor M.